The printing industry has gone through an evolution designed to speed up the printing process and at the same time to lower costs. The film transfer to plate scheme simply exposes light sensitive photographic film with light from a low power source and then uses the processed film to transfer the image to a plate. More efficient plate imaging methods led to the direct imaging of the plates themselves. Although still using relatively low power light sources, such methods required complex plate composition and processing chemistry, and hence a large expense for the plates themselves. In a drive to reduce plate costs, the industry moved to high power laser light sources as a means of thermally exposing the image on the plate. This class of media is composed of inexpensive coatings that require little or no processing to condition the plate surfaces for printing.
In general, raster recording devices are limited in their exposing power by the brightness of the optical source. This limitation is in contrast to a flood exposure system, used to photo-graphically transfer images from a master on to a secondary medium, in which the amount of light can be increased simply by increasing the size or number of sources. Ultimately, the exposing power of any optical system reaches a fundamental maximum, for a given source brightness, once the system aperture and field of view have been filled. Because the field of view of a raster recorder usually extends over only a very small fraction of the total image area, in many cases extending over only a single image pixel element, the maximum exposure limit can be restrictive in terms of exposing power.
In particular, the image transfer process is used in the production of lithographic printing plates. The photo-sensitive emulsions appropriate for coating conventional printing plates are based on photo-polymerization reactions, which require high levels of ultra-violet exposure. The source power required to expose a conventional plate efficiently, with a raster film recorder, is prohibitive using present day technology. A transfer medium is therefore used, composed of silver-halide based emulsions which are much more sensitive to longer wavelength light and require significantly reduced levels of exposure to sensitize. After the image has been generated on the transfer medium, it is used as a photographic mask and copied by contacting it to the printing plate and providing exposure from a high intensity ultra-violet flood lamp.
Lasers are the favoured light source for many raster recording devices because of their inherent high brightness, but they are limited to known lasing materials which impose a number of design restrictions, such as the choice of available wavelengths. In particular, ultra-violet laser sources are much more difficult to manufacture, and are considerably more costly than longer wavelength lasers. Presently, semi-conductor lasers are the most commercially viable laser, in terms of cost per unit emitting power. However, they are only capable of emitting wavelengths in the near infra-red to red portion of the optical spectrum. For this reason, printing plate manufacturers have recently developed printing plates based on thermally induced material changes that are sensitive to high power, near-infra-red (NIR) exposure instead of ultra-violet.
The exposure mechanism of thermally induced media is fundamentally different than photo-polymer or silver-halide processes. The latter processes can integrate exposures without suffering significant reciprocity effects. The former utilize emulsions which react to the thermal load imparted by the exposure, and will undergo a permanent state change, such as ablation, only when a certain temperature threshold has been exceeded. If the thermal load is allowed to dissipate before the threshold has been reached, no change in the reactive material will occur. It is important to consider this effect when designing a raster optical recording system, because some architectures provide varying delays between adjacent lines of raster, which could result in exposure uniformity problems.
Semi-conductor lasers are characterized not only by their wavelength, but also by the form of the emitting aperture. In general, state of the art high power laser diode sources emit from a stripe aperture, which generates a single transverse mode from the width of the strip in one axis, and is multi-modal over its length. NIR lasers of equivalent power (approximately 5 watts), which produce a circular spot, are much more costly than such diode lasers. An example of high power lasers which emit a circular spot is known as a YAG (yttrium aluminum garnet) laser. The brightness of the laser diode is limited by material damage thresholds so that total power can only be increased by enlarging the emitting aperture. The aspect ratio of the emitting aperture can be varied, allowing an increase in total output power to be achieved by increasing the length of the strip rather than increasing its width. The resulting rotational asymmetry imposes restrictions on the design configuration of an efficient raster optical system.
One method of recording high bandwidth and high resolution image data, using an optical source which is extended in one axis, was developed in the 1930's for television applications, and is referred to in the literature as "Scophony" projection. Referring to FIG. 1, the "Scophony" projection system employs an extended source 11 to illuminate an acousto-optic cell 13, with the long axis of the source being parallel to the direction of acoustic propagation. The diffracted light from the acoustic cell 13 is focused on to the recording surface 17, and the image of the acoustic amplitude can be resolved as it traverses the beam. The diffracted light beam is then scanned by mechanical means to sweep across the raster lines of the recording surface 17, and is aligned so that the long axis of the imaged acoustic cell lies along the direction of scanning 19 on the recording surface 17. If the velocity of the scan motion 19 at the recording surface 17 is equal in magnitude but opposite in direction to the velocity of the imaged acoustic motion 23, the image pattern will remain stationary.
In the historical Scophony projection system the recording surface is a planar one and the scanner is a rotating cylinder having a plurality of elongated planar facets orientated parallel to the axis of the cylinder with the axis of the cylinder perpendicular to the direction of propagation of the incident beam. The scan motion produced by the rotating mirror facets is inherently non-linear because the angle of incidence at the recording plane varies along the scan line. Complex corrective optics can be employed to partially compensate for the distortion, but they are expensive and ultimately restrict the number of resolvable pixel elements and the allowable source aperture aspect ratio.
There are many other optical system architectures used to record raster images on to flexible media. One such system is the internal drum scanning system in which a flexible medium is seated against an interior cylindrical mounting surface. A rotating optical element, usually a mirror or prism, which is disposed along the axis of the cylinder, redirects the modulated light beam radially with respect to the cylinder axis, scanning the beam along the cylinder circumference as it spins. The rotating scanner is translated by means of a mechanical carriage transport, which provides the slow scan axis of motion. Many machines in commercial production today employ this basic architecture in one form or another.
Another architecture is the external drum architecture in which a rotating drum carries a light sensitive plate or film clamped or otherwise held against its exterior surface. A writing head moves back and forth along the length of the drum and exposes pixels on the light sensitive recording medium. A major problem with such a system resides in the requirement of having to rotate a large drum, of considerable mass and rotational inertia, at the high speeds necessary to achieve fast recording rates (state of the art systems plot at raster rates in excess of 200 lines per second). Some external drum systems, overcome this problem by maintaining a relatively slow and manageable rotational rate, and exposing the medium with an array of modulated light sources. By recording multiple raster lines in parallel, a high pixel throughput can be maintained, but at the expense of increased system complexity and cost.
A major advantage of the internal drum configuration resides in the fact that it does not require the large mass associated with a drum to rotate. The relatively small axial scan mirror can be mounted directly on to a motor spindle and rotated at high speeds while still maintaining mechanical accuracy. In addition, the configuration does not have inherent distortion as does the planar recording projection system. The beam is directed through the central axis of the lens elements, and the distance to the recording plane is maintained constant throughout the scan motion. This results in a very simple, robust and inexpensive optical system.
However, a single faceted axial optical scanning element, used in an internal drum scanning system, causes the projected image at the recording plane to rotate about the optical axis as the beam scans along the cylinder circumference. If only circular symmetric, single spots are to be projected on to the recording plane, this rotation effect is unimportant. Therefore, circular beam lasers are the natural ones to use with an internal drum scanning system. For more complex systems which use non-rotationally symmetric optical sources, such as linear arrays, compensation is required. U.S. Pat. No. 3,823,276 issued to Maslowski et al, discloses an optical data read system that projects an image of an array of spots from an inner cylinder surface on to a stationary CCD array. In order to compensate for the image rotation induced by the axial scanning element, a rotating dove prism is interposed in the beam path, and rotated at half the scan rate.
If an elongated source is introduced in an internal drum optical recorder, the rotation of the elongated image must be compensated for. This also can also be accomplished by means of a rotating prism synchronized to track the scan rotation. The phase of the rotation compensation can be adjusted to align the long axis with the scan axis, which is the required orientation for Scophony imaging. If the scan rates are tuned properly, the Scophony matching condition can be for a given acoustic rate and optical system magnification.
Accordingly, it is an object of the invention to provide an optical system, which can provide fast, accurate image recording with a relatively high power light source. It is a further object to provide a system which can accommodate a light source of asymmetric proportions, and is capable of exposing high quality raster images on flexible media. It is also an object of this invention to increase the exposing power of an internal drum raster recording device by increasing the extent of the source without degrading the image quality, by means of Scophony imaging. It is a particular object of this invention to facilitate the direct exposure of printing plates without the necessity of producing a transfer medium.